As will be evident from WO 93/20641, it is known to add a prefix to a telephone number in order to identify the type of communication concerned and therewith distinguish a telephony call from a telephone and a telefax. This is, in actual fact, equivalent to using different telephone numbers.
If a telefax is connected to the same telephone number as a telephone, it is possible to use additional signalling to identify when it is the telefax that is called; c.f. U.S. Pat. No. 5,287,402. It is only the most modern telefaxes that are able to deal with such signalling, however.